1. Field of the Invention
The present invention relates to a positive resist composition for immersion lithography, which is used for immersion exposure (immersion lithography), and a method for forming a resist pattern.
Priority is claimed on Japanese Patent Application No. 2006-280201, filed on Oct. 13, 2006, the content of which is herein incorporated by reference.
2. Description of the Related Art
A lithography technique involves, for example, the steps of forming a resist film formed of a resist material on a substrate, subjecting the resist film to selective exposure with radioactive rays such as light and electron beam, through a mask having a predetermined pattern formed thereon, and developing the resist film to form a resist pattern having a predetermined shape on the resist film.
Along with the refinement of semiconductor devices, developments have been made in making the wavelength of an exposure light source shorter and making the numerical aperture of a projection lens higher (higher NA). Thus, a developing apparatus having an NA of 0.84 using, as a light source, an ArF excimer laser having a wavelength of 193 nm has been developed so far. Further, as the wavelength of an exposure light source becomes shorter, a resist material is required to have improved lithography characteristics such as sensitivity to the exposure light source, and resolution which allows reproduction of patterns of fine dimensions. As a resist material satisfying such requirements, a chemically amplified resist which contains a base resin that has variable alkali solubility under action of an acid, and an acid generator that generates an acid upon light exposure, has been used.
At present, as the base resin for a chemically amplified resist used for an ArF excimer laser lithography, or the like, a resin having a constituent unit derived from a (meth)acrylic acid ester on its main chain (an acrylic resin), in view of excellent transparency to light at around 193 nm, or the like is generally used. As used herein, “(meth)acrylic acid” means either or both of an acrylic acid having a hydrogen atom bonded at an α position, and a methacrylic acid having a methyl group bonded at an α position.
The “(meth)acrylic acid ester” means either or both of an acrylic acid ester having a hydrogen atom bonded at an α position, and a methacrylic acid ester having a methyl group bonded at an α position.
The “(meth)acrylate” means either or both of an acrylate having a hydrogen atom bonded at an α position, and a methacrylate having a methyl group bonded at an α position.
For example, Proceedings of SPIE, Vol. 5754, pp. 119-128 (2005) (Non-Patent Document 1) discloses, as a technique for improving the resolution, a lithography method including as step of filling a liquid having a higher refractive index than that of air (hereinafter also referred to as a “immersion medium”) between a projection lens and a specimen for exposure (immersion exposure), a so-called liquid immersion lithography, which is hereinafter also referred to as immersion lithography.
By the immersion lithography, even with the use of a light source having the same exposure wavelength, a high resolution that is equivalent to that in the case of using a light source having a short wavelength or in the case of a high NA lens can be attained, and there is no reduction in focal depth. Further, the immersion lithography can be carried out by using a conventional exposure apparatus. For this reason, it is expected that the immersion lithography will make it possible to form a resist pattern that is low cost, has high resolution, and is excellent in focal depth. Further, in the preparation of a semiconductor device which requires highly expensive facilities, immersion lithography has attracted much attention in the semiconductor industry from the viewpoints of cost and lithography characteristics such as resolution.
The immersion lithography is effective for formation of all sorts of pattern shapes. Further, it is believed that the immersion lithography can be combined with super resolution techniques such as a phase shift method, and a modified illumination method, which have been under investigation recently. At present, a technique in which an ArF excimer laser is mainly used as a light source is being actively investigated as an immersion lithography technique. Further, water has been mainly investigated as an immersion medium at present.
Recently, fluorine-containing compounds have been actively investigated in a variety of fields for their characteristics such as waterproofness and transparency. For example, in the field of a resist material, an approach that involves introducing an acid-labile group such as a methoxymethyl group, a tert-butyl group, and a tert-butyloxycarbonyl group to a fluorine-containing polymeric compound, thereby in order to make the component useful as a base resin for a positive chemically amplified resist, is being carried out at present. However, in the case where this fluorine-based polymeric compound is used as a base resin for a positive resist composition, there are drawbacks, such as generation of a large amount of out-gas after exposure, insufficient resistance to dry etching exposure (etching resistance), and so on.
For example, recently, a fluorine-containing polymeric compound having an acid-labile group containing a cyclic hydrocarbon group has been reported as a fluorine-containing polymeric compound having excellent etching resistance in Proceedings of SPIE, Vol. 4690, pp. 76-83 (2002) (Non-Patent Document 2).